1. Field of the Invention
The present invention relates to a toothed belt, for example, for driving the overhead cam (OHC) shaft of an automobile, and more particularly it relates to a toothed belt having great durability that makes possible a long life-span under severe operating conditions.
2. Description of the Prior Art
Unlike flat belts, V-shaped belts or the like, toothed belts do not slip, and thus have high transmission efficiency. In addition, compared with transmission apparatus such as gears, chains, etc., the toothed belts have the advantage of very little noise. Thus, toothed belts are used for simultaneous drive in such places as automotive overhead cam (OHC) shafts, and their use is increasing.
Conventionally, a toothed belt is constructed, as shown in FIG. 1, by a back surface section 1 and tooth sections 2, both of which are formed from an elastic substance such as rubber, a tooth cloth 3 which covers the surface of the tooth sections 2 and the tooth lands 5 positioned between the adjacent tooth sections 2 to prevent abrasion arising from friction between the belt and the pulleys, and a plurality of core fibers 4, 4, . . . passing through the inside of the back surface section 1 in the longitudinal direction so as to touch the tooth cloth 3.
A toothed belt with such a structure is used, for example, for a simultaneous drive of an OHC shaft of an automobile. The toothed belt for the driving of an OHC shaft moves at high speeds under a heavy load and at high temperatures. In particular, recently, high output automobile engines are being developed, where the toothed belt moves at higher speeds and the engine temperature becomes higher. When the toothed belt is used under such conditions, the abrasion of the tooth cloth which covers the bottom section of the belt is accelerated by being in contact with pulleys, and at the same time, the tooth cloth is deteriorated by being exposed to high temperatures. When the toothed belt is wound over a large number of pulleys, the toothed belt moves with many bends. Because the toothed belt for driving an OHC shaft is used at high temperatures, the elastic substance such as rubber, etc., that is used for the construction of the back surface section 1 and the tooth sections 2 undergoes aging, which gives rise to severe cracking. Because of movement at high speeds under a heavy load, the abrasion of the tooth cloth 3 which touches the pulleys is great, which easily gives rise to cracking, shear, etc. Similarly, a high pressure is applied to the tooth sections 2 engaged with the pulleys, which easily gives rise to cracking, shear, etc., resulting in damage. Moreover, when the toothed belt moves with a number of bends, being wound by many pulleys, the bending sections of the toothed belt increase, so that internal heat is generated by friction between the core fibers 4 and the elastic substance forming both the back surface section 1 and the tooth sections 2, which causes accelerated aging of the core fibers and elastic substance. As described above, the toothed belt for driving an OHC shaft is used under severe conditions, so it has a problem of durability and cannot be operated for a long period of time.
A toothed belt with excellent durability is proposed in Japanese Laid-Open Patent Publication Nos. 62-159827 and 64-87937. The toothed belt disclosed in Japanese Laid-Open Patent Publication No. 62-159827 uses a hydrogenated acrylonitrile-butadiene copolymer (NBR) with a hydrogenation rate of 80% or more as the back surface section 1 and the tooth sections 2, an industrial nylon raw fiber for the tooth clothing 3, having a viscosity relative to sulfuric acid of 2.6 to 2.8, a tensile strength of 5 g/denier or more, and a fineness of single fibers of 5 denier or less as the tooth cloth 3, and moreover, the core fibers 4 are made of twisted bundles of glass fiber filaments with a predetermined twist number, the core diameter with respect to the tooth cloth being in a predetermined range. The structure as described above makes possible remarkably improved durability as well as the stable movement for a long period of time at high speeds under a heavy load.
However, there is a problem such that when such a toothed belt is used for driving an OHC shaft of an automobile, sufficient durability cannot be obtained because of the severe conditions, and the life-span is shorter compared with that of the engine. In particular, recently, higher output of automobile engines are being attempted, so that the toothed belt moves at higher speeds and the engine temperature becomes higher. When the toothed belt is used under these conditions, the abrasion kf the tooth cloth which covers the bottom section of the belt is accelerated by being in contact with the pulleys, and at the same time, the deterioration of the tooth cloth is accelerated by being exposed to high temperatures. When the belt moves, the applied force concentrates at a root section of a tooth and the root section is readily bent, so that the movement of the belt under such conditions gives rise to cracking of the tooth cloth at the root section, and moreover, the tooth section of the belt suffers a loss.
As described above, the toothed belt used in an engine room moves at high temperatures and high speeds, resulting in the damage of the belt due to the deterioration of the tooth cloth.
Also, the inside of the engine room is sealed with a cover, but the seal property is not sufficient. Therefore, water, sand, mud, or dust, etc., (hereinafter, referred to as environmental foreign matter) invade the engine room, and there is the possibility that they may cover the belt together with engine oil or the like in the engine room. When such environmental foreign matter invades between the tooth cloth 3 at the tooth lands 5 and pulleys, the abrasion of the tooth cloth 3 is accelerated. Such an abrasion of the tooth cloth 3 is accelerated faster than that of the tooth cloth 3 occurring under the severe conditions described above, so that there is the possibility that cracking is generated in the tooth cloth 3 of the root section. The abrasion of the tooth cloth 3 is accelerated to cause damage to the core fibers 4 which are in direct contact with the tooth cloth 3 in the early stages. Such conditions result in a loss of the tooth sections 2, a break of the core fibers 4, etc. Since the damage of the belt caused by such environmental foreign matter occurs earlier than that occurring under severe conditions such as high temperatures, it is conceivable that the elements such as thermal fatigue and bending fatigue may have some effects on the damage rate of the belt.
As described above, in the toothed belt for an automobile engine, in order to improve the life-span of the belt, it is important to prevent the damage of the belt caused by the environmental foreign matter. For this reason, it is necessary to improve heat-resistance, resistance to bending, resistance to abrasion, strength, and adhesive property between the core fibers and the back surface section.
In view of the back surface section 1 and the tooth sections 2, since yet a higher output of an automobile engine is further expected, these portions are further exposed to high temperatures. Because of this, the rubber material forming the back surface section and the tooth section is required to have stronger heat-resistance.
It is usually preferred that the diameter of the core fibers 4 may be small in order to suppress the heat generation due to the friction with the rubber material forming the back surface section 1 and the tooth sections 2, the heat generation due to the friction with the tooth cloth 3, and moreover the internal heat generation in the core fibers 4 themselves (heat of friction between each fiber forming the core fibers). However, when the diameter of the toothed belt is made small, the strength of the core fibers 4 themselves is lowered and sufficient tensile strength cannot be obtained. Because of that, it is necessary to use core fibers 4 with a small fineness but with sufficient tensile strength.
The tooth cloth 3 is in direct contact with pulleys and is subject to environmental effects such as those caused by the environmental foreign matter, so that various mechanical characteristics of the tooth cloth 3 have a great effect on the entire characteristics of the toothed belt. Accordingly, if the tooth cloth 3 is not composed of good materials, a good balance of various mechanical characteristics which are necessary for a toothed belt having excellent resistance to the environment cannot be obtained.
Also, when considering the engagement of the tooth cloth 3 with the pulleys, there is a problem that the thickness of the tooth cloth 3 is limited by the diameter of the core fibers 4. In order to obtain an appropriate engagement with the pulleys, the thickness of the tooth cloth 3 and the diameter of the core fibers 4 at the tooth lands must be within a predetermined range. Accordingly, when the thickness of the tooth cloth 3 becomes large, the mechanical strength of the tooth cloth 3 itself improves. However, in view of the engagement with the pulleys, the diameter of the core fibers 4 must be made small. In order to enlarge the thickness of the tooth cloth 3, the diameter of the core fibers 4 must be made small. The bending rigidity of the belt itself is lowered by making the diameter of the core fibers 4 small, so that the bending fatigue property of the tooth cloth 3 at the root section can be reduced. When the diameter of the core fibers 4 is made small, at the tooth lands, the area of contact between the core fibers 4 and the rubber material forming the back surface section 1 and the tooth sections 2, and the area of contact between the core fibers 4 and the tooth cloth 3 become small. On the other hand, the area of contact between the rubber material and the tooth cloth 3, which are in contact with each other between each core fiber 4, increases.
In particular, when the core fibers 4 are made of glass fiber, the elasticity of glass fiber is very low compared with rubber material, so the shock which is generated when the belts are engaged with the pulleys cannot be absorbed by the glass core fibers themselves. However, as described above, when the area of contact between the rubber material and the tooth cloth 3 is increased, the shock with the pulleys is absorbed by the rubber material and the shock applied to the core fibers 4 is alleviated. In the same way, the shock applied to the tooth cloth 3, which is generated by the engagement with the pulleys, is absorbed by the rubber material which is in contact with the tooth cloth 3, so that the durability of the tooth cloth 3 itself improves. When environmental foreign matter intervenes between the tooth cloth 3 and the pulleys, the shock applied to the tooth cloth 3, core fibers 4, and rubber material is further increased. However, the area of contact between the tooth cloth 3 and the rubber material is increased, so that the shock is absorbed by the rubber material and the durability of the tooth cloth 3 and the core fibers 4 is further improve compared with the prior art.